Heat switches are needed to control the heat flow between adjacent stages in an Adiabatic Demagnetization Refrigeration (ADR) process. Heat switches are typically one of two types: those that use a metallic switching element and those that use a gas or fluid switching element. The heat switches that use a metallic switching element are typically mechanical, super conducting or magnetoresistive switches. Heat switches that use a gas as the switching element are called gas-gap switches. The concept is to place two conductive metal plates close to each other and introduce a gas between them to turn the switch On, and remove the gas to turn the switch Off. One of the disadvantages of all of the aforementioned switches is the need for some sort of actuator. Not only does this require ancillary control electronics, software and sensors, these requirements may have a significant thermal impact on the system. Actuators require wiring or drive shafts which conduct heat and dissipate heat when used. This could be a concern when operating in a cryogenic environment.
An ADR stage produces cooling (or heating) by the interaction of a magnetic field with the magnetic spins in a paramagnetic salt. Magnetizing the salt produces heating, and demagnetizing the salt produces cooling. A conventional “single-shot” ADR consists of a “salt pill” containing the magnetic salt, a superconducting magnet, and a heat switch. The salt pill is located in the bore of the magnet, and the heat switch links it to a heat sink. Regardless of the initial conditions, the refrigeration cycle consist of the following steps: First, the salt pill is magnetized, causing it to warm up. Second, when its temperature exceeds that of the heat sink, the heat switch is powered into the on state. Third, the salt continues to be magnetized, generating heat which flows to the sink. This continues until full field is reached, which necessarily is strong enough to significantly align the spins and suppress the entropy of the salt. Fourth, at full magnetic field, the heat switch is deactivated to thermally isolate the salt from the heat sink. Fifth, the salt is demagnetized to cool it to the desired operating temperature. In general, the salt will then be receiving heat from components parts. The heat is absorbed and operating temperature maintained by slowly demagnetizing the salt at just the right rate. Heat can continue to be absorbed until the magnetic field is reduced to zero, at which point the ADR has run out of cooling capacity.
Over the last few years there has been a growing need for more advanced ADR cooling technology. The space industry has been a pioneer in this technology because ADRs are the only low temperature (below 0.2° K) refrigeration technology that does not use any fluids, and therefore does not have the design constraints imposed by gravity. Recently ADRs have been developed for commercial use particularly in the high resolution, high efficiency, x-ray spectrometer industry. The trend in developing ADRs is toward using multiple cooling stages as this arrangement allows for greater efficiency, by reducing parasitic heat flows within the refrigerator, and greater operating temperature range. In this process each stage is thermally connected to the next via a heat switch. Thus, for low temperature ADR systems there exists a need for a heat switch that is capable of conducting heat at sub-Kelvin temperatures (down to approximately 200 mK) and is capable of being turned off at those temperatures. Existing gas-gap switches that use a getter to remove the conductive gas from the switch body cannot meet the latter requirement. As the benefits of using ADRs become more widely known it is anticipated that a wider array of industries will take advantage of this efficient cooling process.
The present invention, in a more general application, provides an easy way to cool something below room temperature and then automatically thermally isolate it at a low temperature. This is made possible by providing a heat switch that is capable of conducting heat at temperatures ranging from 0.25° K to above room temperature. One of the heat switches in this multistage process must be conductive in the 0.25° K to 0.3° K range. All of the heat switches must be capable of switching Off in a short period of time (1-2 minutes), and when Off to have a very low thermal conductance. Currently, no heat switches are capable of meeting these requirements. Superconducting switches have too much conductance in the Off state. Mechanical heat switches have too little conductance in the On state. Finally, traditional getter-activated gas-gap heat switches have long turn-off times. Thus there is a need in the industry for a passive gas-gap heat switch that can facilitate the heating/cooling of a device in the 0.25° K to above 1° K range in a manner such that the heat switch does not require long turn-off times.